Many light-emitting devices have a light-emitting diode or similar device located therein that serves as a light source. A light-emitting diode requires a forward current to pass through it in order to emit light. Therefore, when a light-emitting device is connected to a circuit or the like, the polarity must be correct; otherwise, no light will be emitted.
Many light-emitting devices have markings to indicate cathode and anode in order to connect them correctly. However, as light-emitting devices get smaller, it is difficult to see these markings, which can lead to incorrectly connected light-emitting devices.